In a thermal desalination system, comprising a multi-effect evaporator having a plurality of effects, feed water is supplied to a first effect where it is heated, for example by steam passing through heating tubes. Water vapour is evaporated from the feed water and is passed through heating tubes in a subsequent effect, and a series of effects can be chained in this way. The water vapour is condensed and removed as product water. Depending on the configuration of the system, steam may be introduced into the first or last effect.
A limiting factor in the operating temperature of the system, and hence in the efficiency of recovery of the system and its cost-effectiveness, arises from the presence in the feed water of dissolved minerals. In sea water, a particular problem is the presence of calcium and carbonates which form calcium carbonates (CaCO3). At higher temperatures, the calcium carbonate is more likely to be precipitated, and thus the operating temperature of the final effect is limited by the proportion of dissolved minerals. Conventionally this problem is approached by either operating the systems at low recovery rates and/or relatively low temperature or by implementing a pre-treatment stage of the feed water with chemicals to reduce the dissolved mineral content, which is undesirable.